The present invention relates to a driving method and a driving circuit for a stepping motor which drives a carriage and the like of a scanner, for example, in an image forming apparatus and the like.
In general, for example, in the driving method for a stepping motor which drives a carriage and the like of a scanner, for example, a micro step driving is performed. That is, by adopting the micro step driving, the number of partitions is increased or decreased in relation to the vibration generated in a low velocity area of the stepping motor, thereby allowing a peak current value to be variable and decreasing the vibration.
Now, in FIG. 1, an ideal value of a phase current waveform (assumed to be of xe2x85x9 partition herein) in the micro step driving is shown, and it will be described below.
In general, regarding a current control, a chopping system is adopted, in which an ON state for causing current to flow for a target current value and an OFF state for not causing current to flow are controlled by a higher frequency (20 to 50 KHz) than a motor driving frequency.
In the current control by this chopping system, a rise through rate and a fall through rate of a motor driver are constant.
Hence, according to characteristics of the rise through rate and the fall through rate of stepwise waveforms as shown in FIG. 1, the rise through rate and the fall through rate are controlled by current waveforms as shown in FIGS. 2 to 9. That is, FIG. 2 shows a waveform when a current waveform at a point (X) of FIG. 1 is controlled by the rise through rate xe2x80x9clargexe2x80x9d and the fall through rate xe2x80x9clargexe2x80x9d. FIG. 3 shows a waveform when a current waveform at a point (Y) of FIG. 1 is controlled by the rise through rate xe2x80x9clargexe2x80x9d and the fall through rate xe2x80x9clargexe2x80x9d.
Also, FIG. 4 shows a waveform when a current waveform at a point (W) of FIG. 1 is controlled by the rise through rate xe2x80x9clargexe2x80x9d and the fall through rate xe2x80x9clargexe2x80x9d. FIG. 5 shows a waveform, in which a current waveform at a point (Z) of FIG. 1 is controlled by the rise through rate xe2x80x9clargexe2x80x9d and the fall through rate xe2x80x9clargexe2x80x9d
In the current waveforms shown in these FIGS. 2 to 5, since the rise through rate is xe2x80x9clargexe2x80x9d, the rise of current is quick. Hence, an overshoot and an undershoot are generated largely so as to make a ripple of the current large.
Particularly, as evident from FIGS. 3 and 4, when a step difference of the current is small, the ripple of the current becomes larger than the step difference of the current and is not controlled correctly to the stepwise waveforms (Y) and (W). Also, as shown in FIGS. 2 and 5, when the step difference of the current is large, since the ripple of the current is smaller than the step difference of the current, it relatively comes closer to the stepwise waveform. However, the current ripple does not differ from FIGS. 3 and 4. The current being unable to be controlled to the correct stepwise waveforms due to the influence of the ripple of the current is a cause of generating vibration without being able to correctly control the rotation of the motor.
On the other hand, FIG. 6 shows a waveform when the current waveform at the point (X) of FIG. 1 is controlled by the rise through rate xe2x80x9csmallxe2x80x9d and the fall through rate xe2x80x9csmallxe2x80x9d. FIG. 7 shows a waveform when the current waveform at the point (Y) is controlled by the rise through rate xe2x80x9csmallxe2x80x9d and the fall through rate xe2x80x9csmallxe2x80x9d.
Also, FIG. 8 shows a waveform when the current waveform at the point (W) is controlled by the rise through rate xe2x80x9csmallxe2x80x9d and the fall through rate xe2x80x9csmallxe2x80x9d. FIG. 9 shows a waveform when the current waveform at the point (Z) is controlled by the rise through rate xe2x80x9csmallxe2x80x9d and the fall through rate xe2x80x9csmallxe2x80x9d.
In FIG. 6, since the step difference of the current is large and the rise through rate is xe2x80x9csmallxe2x80x9d, the rise of the current is delayed and takes a delay time of td1 and does not follow the current waveform (X) of FIG. 1. In FIG. 9, since the step difference of the current is large and the fall through rate is xe2x80x9csmallxe2x80x9d, the fall of the current is delayed and takes a delay time of td2 and does not follow the current waveform (Z) of FIG. 1. The generating of the delayed time in such a manner means that the control of the motor is delayed. The current being unable to be controlled to the correct stepwise waveforms due to the influence of the delay time of the current is a cause of generating vibration without being able to correctly control the rotation of the motor.
In FIGS. 7 and 8, since the step difference of the current is small, even if the through rate is small, the waveform follows stepwise thereby it is possible to control the current. Also, since the rise through rate or the fall through rate is small, the ripple is also made small.
In this way, in the method for driving a stepping motor according to the prior art, since it is a control method by allowing the rise through rate and the fall through rate of the current to be constant, even if a waveform can be controlled in a part of the stepwise waveforms, in other parts thereof, a large ripple or a large delay time in a follow-up property of the current control has been generated. Thus, it is not possible to control the waveform to a correct stepwise waveform and the generation of the vibration has been caused.
Now, in FIGS. 10A and 10B, a relationship between a phase A and a phase B in the case where a relationship between a micro step driving current waveform and a drive through rate is used in the conventional rise through rate xe2x80x9clargexe2x80x9d and the fall through rate xe2x80x9clargexe2x80x9d is shown, and it will be described below.
The driving current waveform of the phase A at a point (P) of FIG. 10A denotes FIG. 4. The driving current waveform of the phase B at a point (P) of FIG. 10B denotes FIG. 2.
At a point (X) of the phase B, the current ripple is xe2x80x9clargexe2x80x9d and yet it follows the stepwise waveform. However, at a point (W) of the phase A, since the ripple is large, it is unable to follow the stepwise waveform. This shows that, at a point (W) of the phase B, a rotational angle can maintain a position, but at a point (X) of the phase A, the rotational angle cannot maintain the position. In the micro step driving of the stepping motor, the rotational angle is controlled by a value of current caused to flow to the phase A and the phase B. However, at the point (P), the angle position of the phase A fluctuates and therefore it shows that the angle position of the stepping motor fluctuates. The generating of the fluctuation of the rotational angle shows that the vibration is generated in the stepping motor.
The driving current waveform of the phase A at a point (Q) of FIG. 10A denotes FIG. 5. The driving current waveform of the phase B at a point (Q) of FIG. 10B denotes FIG. 3.
At a point (Z) of the phase A, the current ripple is xe2x80x9clargexe2x80x9d and yet it follows the stepwise waveform. However, at a point (Y) of the phase B, since the ripple is large, it is unable to follow the stepwise waveform. This shows that, at a point (Z) of the phase A, the rotational angel can maintain the position, but at a point (Y) of the phase B, the rotational angle cannot maintain the position. Due to the same reason as the fluctuation of the rotational angle at the point (P), at the point (Q) also, the rotational angle fluctuates and the vibration is generated in the stepping motor.
On the other hand, in FIGS. 11A and 11B, the relationship between the phase A and the phase B in the case where the relationship between the micro step driving current waveform and the drive through rate is used in the conventional rise through rate xe2x80x9csmallxe2x80x9d and the fall through rate xe2x80x9csmallxe2x80x9d is shown, and it will be described below.
The driving current waveform of the phase A at a point (P) of FIG. 11A denotes FIG. 8. The driving current waveform of the phase B at a point (P) of FIG. 11B denotes FIG. 6.
At a point (W) of the phase A, the ripple follows the stepwise waveform, but at a point (X) of the phase B, since the rise of the current is delayed, it is unable to follow the stepwise waveform. This shows that, at the point (W) of the phase A, the rotational angle can maintain the position, but the point (X) of the phase B, the rotational angle cannot maintain the position. In the micro step driving of the stepping motor, the rotational angle is controlled by the current caused to flow to the phase A and the phase B. However, at the point (P), the angle position of the phase A fluctuates, therefore it shows that the angle position of the stepping motor fluctuates. The generating of the fluctuation of the angle position shows that the vibration is generated in the stepping motor.
A driving current waveform of the phase A at a point (Q) of FIG. 11A denotes FIG. 9. A driving current waveform of the phase B at a point (Q) of FIG. 11B denotes FIG. 7.
At a point (Y) of the phase B, the ripple follows the stepwise waveform, but at a point of (Z) of the phase A, since the fall of the current is delayed, it cannot follow the stepwise waveform. This shows that, at the point (Z) of the phase A, the rotational angle can maintain the position, but the point (Y) of the phase B, the rotational angle cannot maintain the position. Due to the same reason as the fluctuation of the rotational angle at the point (P), at the point (Q) also, the angle position fluctuates and the vibration is generated in the stepping motor.
Incidentally, in Japanese Patent Application KOKAI Publication No. 9-219995, a technology is disclosed, in which the decrease in the current at the time of off during the chopping operation for the constant current to be supplied at the time of the micro step driving is performed by a combination of high speed attenuation and low speed attenuation. However, the through rate of the current at the time of the current increase and current decrease is not allowed to be variable corresponding to the magnitude of the step difference of the current stepwise waveform at the time of the micro step driving.
The present invention has been carried out in view of the above described problems and it is an object of the present invention to provide a driving method and a driving circuit for a stepping motor, in which, similarly to a micro step driving, when there is a stepwise step difference between current waveforms, a rise through rate is changed and controlled by matching the step difference at an increase time of a value of current, and a fall through rate is changed and controlled by matching the step difference at a decrease time of the value of current and further, a through rate matched with the step difference is used at a point of change of the current and from after the current becomes a constant value, the rise/fall through rates are controlled so as to be xe2x80x9csmallxe2x80x9d, thereby obtaining a current waveform in which the ripple is reduced and follows the stepwise waveform.
In order to achieve the above described object, a method for driving a stepping motor which performs the micro step driving of the present invention comprises the steps of: making a rise through rate variable by matching a step difference related to a change in a value of current when the value of current caused to flow in the stepping motor is increased in the micro step driving; and making a fall through rate variable by matching a step difference related to a change in a value of current when the value of current caused to flow in the stepping motor is decreased in the micro step driving.
Further, there is provided a method for driving a stepping motor by a driving circuit of a stepping motor having a first transistor group which controls a rise through rate of a driving current caused to flow in the stepping motor, a second transistor group which controls a fall through rate of the driving current caused to flow in the stepping motor, and a transistor control circuit which controls ON/OFF states of the first and second transistor groups, comprising the steps of: variably controlling a rise through rate by controlling the ON/OFF states of the first transistor group by matching a step difference related to a change in a value of current by the transistor control circuit when the value of current caused to flow in the stepping motor is increased in the micro step driving; and variably controlling a fall through rate by matching the difference of a value of current by controlling the ON/OFF states of the second transistor group by matching the step difference related to the change in the value of current by the transistor control circuit when the value of current caused to flow in the stepping motor is decreased in the micro step driving.
Further, the driving circuit of the stepping motor of the present invention performs the micro step driving and comprises: a first transistor group which controls a rise through rate of a driving current caused to flow in the stepping motor; a second transistor group which controls a fall through rate of a driving current caused to flow in the stepping motor; and a transistor control circuit, in which, by controlling ON/OFF states of the first and second transistor groups, the driving current caused to flow in the stepping motor is made variable, thereby variably controlling the rise through rate and the fall through rate.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.